131blastomeres (see above). For the ongoing cell cycle, a primary function of the ana-
phase aster appears to be furrow induction at the site of overlap of microtubules
from each spindle pole, presumably by providing furrow induction signals such as
CPC factors as discussed above. A second function, indicated by the slight improve-
ment in spindle alignment observed in anaphase, seems to be to add precision late
in the cell cycle to the orientation of the alignment of the spindle acquired earlier in
the cell cycle. The precise mechanistic nature of this late alignment has not been
studied, but presumably involves length-dependent internal pulling forces and/or
(because anaphase microtubules do reach the cortex) cortex-sensing mechanisms.
The second function, fine-tuning of orientation, can be explained by a cortex-
sensing mechanism. But, how does the spindle largely acquire its future orientation
early in the cell cycle, even prior to its ability to sense the cortex and possibly cell
shape? A key concept for understanding this mechanism of early spindle alignment
is that, in the rapidly cycling blastomeres of the early embryo, cytoskeletal struc-
tures involved in cell division exhibit a degree of overlap between cell cycles. Thus,
the spindle for a given cell cycle is starting to form at a time in which telophase
astral microtubules of the previous cell cycle are still carrying out essential cell divi-
sion functions. In particular, telophase astral microtubules, in addition to executing
the above-described functions on furrow induction and spindle alignment fine-
tuning for an ongoing cell cycle, provide cues for the early orientation of the meta-
phase spindle for the following cell cycle (Wühr et al. 2010 ).
This influence of furrow orientation from one cycle to the next appears to depend
on a zone of microtubule exclusion that forms at the site of anaphase astral micro-
tubule overlap, which develops in the plane in which the future cytokinetic furrow
will cleave (Figs. 4.2, 4.4, and 4.5). As mentioned above, metaphase asters are rela-
tively small and do not reach the cortex. On the other hand, anaphase asters grow
dramatically to fill in the entire space of blastomeres and contact the cortex, where
they provide signals to induce furrowing during cytokinesis. As telophase asters
from opposite sides of the spindle reach the midzone, a microtubule-free (microtu-
bule interaction) zone appears which clearly delineates the site of the forming fur-
row. This zone of microtubule exclusion generates a “dome”-shaped aster, where
the sides of the dome correspond to the new spindle’s long axis, aligned parallel to
the plane of the forming furrow (Wühr et al. 2010 ). This dome shape, with microtu-
bules being longer on the side of the spindle opposite the microtubule interaction
zone at the furrow (Figs. 4.4 and 4.5), generates an asymmetric force that moves the
MTOCs toward the center of the future daughter cells and aligns the nascent spindle
along the axis parallel to the forming furrow. The mechanism underlying the
formation of the microtubule interaction zone is poorly understood. Embryos
mutant for the zebrafish maternal-effect gene motley, which encodes an isoform of
the CPC component survivin, do not exhibit a microtubule interaction zone at the
furrow. In these mutants, anaphase astral microtubules instead cross the furrow
boundary from opposite directions to generate a diffuse region of overlap (Nair
et al. 2013 ).
The series of cellular events that result in early spindle alignment in zebrafish
and Xenopus embryos (and possibly other vertebrates with large blastomeres) can
4 Vertebrate Embryonic Cleavage Pattern Determination